Reductive amination is an important commercial process in which reactants such as alcohols, ketones, aldehydes, or the like are converted to amines. The reactants form imine intermediate(s). These are then reduced to form amine products. For instance, reductive amination techniques can be used to react monoethanolamine (MEA) and ammonia to form amine reaction products.
Reductive amination generally is carried out in the presence of a catalyst in a reducing environment. A typical catalyst includes at least one catalytically active metal having an activity for reductive amination. Examples of such metals include Ni, Co, and/or Cu. Rhenium (Re) is also incorporated into these catalysts. Although Re may have catalytic activity on its own, it is believed that Re functions as a promoter to enhance the catalytic activity of the Ni, Co, and/or Cu.
Fresh catalyst has a certain level of activity and selectivity for catalyzing reductive amination. The selectivity and/or activity may diminish over time as the catalyst ages and is used. At some point, the catalyst is deemed to be spent and is replaced. Other times, the catalyst may still have activity and selectivity, but the manufacturing plans have changed so that the catalyst is no longer needed.
To be effective for reductive amination, catalysts incorporate relatively large amounts of Re. For example, weight loadings greater than 0.5 weight percent Re on a metals basis based on the total weight of the catalyst typically are used. Thus, a significant amount of Re is used to carry out reductive amination on a commercial scale. Other kinds of catalysis use much less Re under different conditions. For instance, ethylene oxide (EO) catalysis uses much lower amounts of Re, the catalysis occurs in an oxidizing environment, and the catalyst often includes significant amounts of Ag. Thus, in a typical recovery of Re from EO catalysts, the catalyst was used in an oxidizing environment, and the Re is to be recovered selectively relative to substantial amounts of Ag.
Rhenium also is an extremely expensive and rare metal. Rhenium costs have gone as high as $10,000 per kg. Based on the amount of Re being used and its cost, it is highly desirable to recover and re-use Re from spent catalysts. More particularly, it is desirable to recover Re selectively relative to other catalyst constituents to avoid undue contamination and thereby simplify recycling of Re.
Extraction techniques have been used to recover Re from spent catalysts. Unfortunately, U.S. Pat. No. 7,763,096 reports that aqueous extraction has poor selectivity for Re. Significant amounts of other catalyst constituents are extracted along with the Re so that the Re is contaminated with these other materials. US 2009/0148361 also reports that aqueous extraction can recover both Re and metal catalysts non-selectively and thereby hinder or complicate recovery of the Re.
There remains a strong need to selectively recover Re from used reductive amination catalysts. In particular, there is a strong need to recover Re selectively relative to Ni, Co, and/or Cu from used reductive amination catalysts